The present invention relates to a cooling structure which provides heat transfer between electronic components and a flowing coolant including a cooling hat comprising a coldsheet, a plurality of manifold layers and springs. Each manifold layer includes a branched hierarchy of conduits for coolant supply and coolant return. A highly parallel and streamlined flow achieves ample fluid flow with a small hydraulic differential pressure. The springs, when present, force the cooling hat into intimate contact with the surface to be cooled.
High speed computers and other fast electronic systems often require assemblies of many integrated circuit chips where each chip contains many active devices, and many chips are spaced closely together. During normal operation the devices dissipate a very large power density, especially bipolar transistor devices. Proper electronic operation of the devices necessitates a cool operating temperature which, in turn, requires adequately cooling of the power density. Conversely the maximum allowable operating temperature of the devices and integrated circuit chips in combination with the limited cooling capability presently available limit the allowable power density, circuit density and system speed. Improved device and integrated circuit chip cooling results in increased permissible power density, circuit density and system speed.
The prior art contains many forms of cooling. One form of cooling which has been proposed is the use of a metal plate held against circuit chips by springs, as is disclosed by Cutchaw in U.S. Pat. No. 4,381,032. A heat exchanger incorporating a deflectably movable diaphragm in forced engagement with an integrated circuit package is disclosed by Cutchaw in U.S. Pat. No. 4,341,432. Another form of heat exchanger for cooling electronic circuits provides for passages within which liquid coolant is circulated, the coolant contacting a flexible wall which is urged against the circuitry to be cooled as shown by Wilson et al, in U.S. Pat. No. 4,072,188. Another form of heat exchanger employs coated metallic dendrites which are held by springs against a circuit chip as disclosed by Babuka et al in U.S. Pat. No. 4,254,431. Yet another form of heat exchanger employs a pillow structure formed of a film and filled with a thermal liquid material for extracting heat from an electric circuit, as is disclosed by Spaight, in U.S. Pat. No. 4,092,697. Also a malleable dimpled wafer is deformed by pressure between a heat source such as an electronic circuit and a heat sink, as is disclosed by Rhoades et al in U.S. Pat. No. 4,151,547. Other U.S. patents showing a single layer of material interposed between a circuit chip and a cooling device are Steidlitz, U.S. Pat. No. 4,069,497; Balderes et al, U.S. Pat. No. 4,233,645; Yoshino et al, U.S. Pat. No. 4,546,409; Kohara et al, U.S. Pat. No. 4,561,011; Hassan et al, U.S. Pat. No. 4,607,277; Watari, U.S. Pat. No. 4,612,601; Ostergren et al, U.S. Pat. No. 4,639,829; and Meagher et al, U.S. Pat. No. 4,462,462. The use of liquid and reentrant cavities at a thermal interface is disclosed by Pease, Tuckerman and Swanson in U.S. Pat. No. 4,567,505. The use of a composite structure of a conformal coating plus liquid at a thermal interface is disclosed by Berndlmaier et al, in U.S. Pat. No. 4,323,914. A theoretical discussion of cooling considerations is presented in an article in the IEEE Electron Devices Letters, "High Performance Heat Sinking For VLSI" by D. B. Tuckerman and R. F. W. Pease, Vol. EDL-2, No. 5, May 1981. Broadbent, U.S. Pat. No. 4,602,314 and Sherman, U.S. Pat. No. 4,258,411 disclosed a flexible thermally conductive body disposed between a semiconductor device and a heat sink. U.S. Pat. No. 3,626,252 discloses a silicone grease loaded with thermally conductive particles disposed between a heat sink and an electronic device.
A well known thermal joint is a single thin layer of oil disposed between a chip and a cooling means. A crude thermal joint is a dry joint between a chip and a cooling means. Such a thermal joint provides contact only at tiny asperities, and everywhere else there are tiny air gaps and poor thermal conduction.
The foregoing cooling systems are inadequate for modern electronic systems, particularly bipolar chips packaged closely together in a Multi Chip Module. Therefore, a piston-linkage cooling system has been used. One example of such a cooling system is described in the IBM Journal of Research and Development, Vol 26 No. 1, January 1982. In the described arrangement approximately 100 bipolar semiconductor chips are each bonded face down. Numerous small solder balls connect each chip to a common printed circuit. The solder ball is a Controlled Collapse Chip Connector (so-called C4 connectors), and the printed circuit is a Multi-Layer Ceramic substrate. Adjacent to these chips is a cooling hat. Each chip is adjacent to a small piston disposed in a socket in a water cooled metal block. During operation, each chip generates heat which is removed. The heat is conducted from the back of the chip, across a small gap to the tip of the piston, along the length of the piston, across a gap to the socket, through the metal block, and finally is removed by convection into the flowing water. In some modifications the piston tip is made flat for better thermal contact with the chip, there is oil between the chip and the piston, and there is thermal paste between the piston and the socket. The modifications provide a certain degree of improved cooling ability.
The piston is designed for movement within its socket to compensate for manufacturing tolerances and thermal distortions. To compensate for chip height variations, the piston is made to slide in the socket in a direction perpendicular to the chip surface. To compensate for chip tilt variations, the piston is designed to tilt within the socket. To compensate for lateral distortions (due to non-uniform thermal expansion), the piston tip is able to slide laterally over the chip surface or alternatively to slightly rotate or slide laterally in its socket. The various compliance modes prevent chip-to-chip variations from causing large stresses and hence damage. However, achieving the compliance modes requires sufficient clearance between the chip and the piston, and between the piston and the socket.
The prior art cooling schemes contain short comings and limitations. In order to effectively remove heat from high power density chips, where many chips are closely packaged on a multichip module, a "tight" thermal path is required from each chip to the coolant. The tight thermal path requirement conflicts with a "loose" path requirement such as is required for the above described compliance modes. In order to protect the fragile C4 connectors, the cooling system must not apply large stresses. Unfortunately, manufacturing variations result in significantly different chip heights and tilts. Also, differential thermal expansion significantly distorts the geometry of the system components so that a completely rigid system would develop excessive stresses. Temperature changes cause thermal expansion or contraction of the chips, substrate and cooling hat. The thermal expansion depends on the material involved, and is generally different for each element. During start up and cool down, there is non-uniform temperature and expansion, which also causes unequal thermal expansion and resultant thermal distortions. If not compensated, damaging stresses may develop at interconnections between the elements. For example, unequal thermal distortion parallel to the substrate surface will cause shear stress and eventual failure of the C4 balls. Such a failure mode must be prevented.
As electronic systems continue to advance, the piston linkage and other cooling systems noted above become inadequate. In some cases, there is too much thermal resistance. In still other cases, the cooling does not provide adequate compliance to counteract variations and distortions. In still other cases, the cooling is excessively complex when applied to a Multi Chip Module containing many chips.
One example of the general problem is the design conflict manifest in piston cooling. To improve heat transfer typically requires tighter clearance, tolerance, and smoothness (from chip to piston tip, and from piston to socket). By contrast, adequate motion and economical manufacturing favor a design with looser clearance, and the like.
One partial solution is to use oil or thermally enhanced paste in the gaps between elements (i.e., between the chip and the piston, or between the piston and the socket). Another partial solution is to design a more sophisticated geometry in which the piston and block are reshaped to increase their contact area. For example, the piston may be reshaped to increase the area adjacent to the metal block. Nevertheless these partial solutions while beneficial do not fully resolve the design conflict.
A single chip module in combination with a printed circuit board may also be employed to package many chips close together. An example is a dual in-line package containing one chip, attached leads, and a plastic housing. The single chip modules are mounted on a common printed circuit board. Some applications might employ single chip modules with printed circuit boards to achieve close chip packing density and high chip power density. Even using such designs, when there is very high performance, a design conflict is manifest between the requirements of a "tight path" for high thermal conductivity and a "loose path" for mechanical compliance of the cooling system element.
Additional prior art concerns the use of cold plates to transfer heat between a cooling fluid and electronic components. Typically a cold plate consists of a thick metal plate with fluid flowing metal piping bonded to the back side of the plate. Heat producing electronic components are thermally coupled to the front side of the plate. The thermal coupling is enhanced by the use of a thermally conductive material disposed between the components and the plate such a material is thermally conducting fine grains dispersed in grease or silicone.
Another cold plate construction includes a thick copper plate with water flow passages machined directly into the plate. The passages are closed by an additional plate bonded to the machined plate.
Typically, electronic components are individually mounted on the cold plate and each component has a relatively flexible electrical connection to other components. In other cases, the components are first assembled on a single circuit board. Then the circuit board and component assembly are urged into intimate contact with the cold plate. Cold plates have limitations in cooling multichip modules. There is a design conflict between a tight path needed for good thermal contact, and a loose path needed to protect the electrical connections of the components to the rigid substrate.
Flexible versions of a cold plate exist, for instance, in the form of a metal sheet with a plurality of flexible metal bellows, one bellows associated with each component to be cooled. A thermal joint couples the bellows to a respective electronic component. Fluid jets flow through each bellows to remove the heat transferred from the component. In order to provide good heat transfer, each fluid jet requires considerable flow. Therefore, small pipes are used to connect the flow in series from bellows to bellows. The drawback to a multiple bellows cold plate is the requirement of construction involving considerable labor and cost and the assembly and bond lines are vulnerable to fluid leakage. If many electronic components are located in close proximity to each other, the assembly and bonding process becomes more difficult. Additionally, the fluid flow system through the corrugated bellows requires considerable fluid pressure.
U.S. Pat. No. 4,730,666 issued to Flint et al, (particularly in FIGS. 3, 9 and 10), teaches a cooling hat. The front layer of the cooling hat is a large flexible metal sheet which includes fins and grooves for enhanced heat transfer.
There is also prior art in the field of fluid logic systems and their construction. Fluid logic gates have been devised which can be constructed from cavities and passages in plates. These plates can be efficiently fabricated by molding or chemical machining. A single plate can have many such gates connected together. By laminating multiple plates, it is possible to construct large fluid logic systems. The construction produces an "integrated circuit for fluid logic" often abbreviated "fluid-IC". The fluid-IC construction facilitates manufacturability of fluid logic.
The present invention concerns a cooling structure which provides excellent heat transfer between electronic components such as integrated circuit chips and a flowing coolant, particularly water, as well as excellent manufacturability and low cost fabrication and assembly. The cooling hat also provides compensation for manufacturing variations in substrate height, tilt, curvature and camber. Depending on the application, there may also be manufacturing variations of chip height and chip tilt as well as thermally induced distortions. The invention also reduces stress on electronic connections between a chip and a substrate, particularly stress caused by the manufacturing variations listed above. The cooling is achieved by using an ample coolant flow with a low pressure drop with the coolant reliably sealed from the electronic components.
The term "compliance" will be understood to refer to "compensation for variations" and "reducing stress" of the components to be cooled and the cooling system itself.
The following description and drawings refer to water cooling for integrated circuit chips mounted on a horizontal printed circuit board or substrate. However, the invention also applies to other electronic components, substrate orientations, and fluids, as well as to transferring heat between non-electronic components. For example, the invention is applicable to cooling individually packaged chips on a printed circuit card or board. The invention is also useful for transferring heat from a hot fluid to cool components.
Patent applications entitled "Thermal Joint" and "Convection Transfer System", assigned to the same assignee as the present application, also concerning heat transfer and cooling are being filed concurrently with the present application and are incorporated herein by reference.